Optical networks, such as the Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH) transport networks and Optical Transport Network (OTN) are well known. Through the use of Wavelength Division Multiplexing (WDM) or Dense WDM (DWDM), these optical networks are able to meet the bandwidth and capacity requirements of many new telecommunications services.
However, telecommunications providers have, in the past, deployed such optical networks after considerable planning through manual processes taking many months. Typically, the network architecture would be designed, equipment ordered, installed, tested and then connections provisioned across the network. The use of ring topologies and optical cross connects has brought about the ability to set up end-to-end connections without manual intervention and to re-route connections upon failure. However, such topologies are still largely pre-planned, difficult to scale and generally unsuitable for providing on-demand services. Furthermore, interoperability between different vendor's equipment and different telecommunications providers has been problematic.
There is now significant momentum in the telecommunications industry to migrate towards a more flexible optical network employing a mesh topology where there is no need to verify every possible photonic path at design time. It is desired that such a network be equipment vendor and telecommunications provider neutral so that a telecommunications provider may deploy equipment from multiple vendors and so that connections may be automatically provisioned through multiple telecommunications provider's administrative domains. If such flexibility can be achieved, future optical networks will have many advantages including improved reliability, utilisation efficiency and the ability to provide new services including on-demand services such as Internet access, Virtual Private Networks (VPNs), Digital Video Broadcast (DVB) and so on.
It is expected that future networks will employ purely photonic switches as well as optical switches with electronic cores. Optical switches with electronic cores (OSs) perform optical-electronic-optical (OEO) conversion in that received optical signals are converted into electronic signals which are electronically switched and then converted back into optical signals for onward transmission. In contrast, purely photonic switches (PSs) switch wavelengths without OEO conversion. PSs may be implemented using various well-known devices including liquid crystal and Micro Electromechanical Systems (MEMS) devices. Henceforth in this document, an optical network which uses PSs in some or all of its nodes will be referred to as a photonic network. OSs and PSs will be collectively referred to a Cross Connects (XCs). Note that a single XC may be capable of functioning as either a OS or PS and this functionality may be dynamically selected.
With the introduction of meshed networks with optical flexibility points—ie XCs—routing of end-to-end connections becomes important. Routing protocols, such as Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS), which are known in the Internet Protocol (IP) domain are being adapted to provide mechanisms for routing in the optical domain. Typically, such routing protocols use algorithms to find the lowest cost or lowest distance path to route the connection through the network.
However, routing in photonic networks must take account of peculiarities of the optical transmission media which are quite unlike electronic transmission media used in conventional electronic IP networks. Various photonic effects impair the optical signal being transmitted and limit the maximum reach of the optical transmission media. These effects include power loss, noise, chromatic dispersion, Polarisation Mode Dispersion (PMD) and non-linear effects such as Cross Phase Modulation (XPM), Four Wave Mixing (FWM) and others. When an optical signal is routed through multiple consecutive PSs, the signal impairment accumulates. There comes a point where the cumulative impairment of the optical signal reaches an unacceptable level in terms of signal quality or bit error rate (BER). This means that a path computed by a routing algorithm may not be viable because it may contain a path segment through multiple consecutive PSs in which the cumulative photonic effects impair the optical signal at an unacceptable level. The problem is exacerbated by the fact that these photonic effects depend not only on the static configuration and specifications of the optical transmission media, but also on the dynamic state of existing connections provisioned across the network. The addition or removal of a wavelength from a transmission link will change the photonic effects that occur not only at that link but also the cumulative impairment that occurs at further transmission links.
One known approach to solving this problem is to design the photonic network to include “optical islands” in which all paths through the optical islands are designed to be optically viable in all circumstances. By employing only OSs at the boundary nodes of the islands, any path computed by a routing algorithm (possibly through multiple optical islands) will be guaranteed to be optically valid. However, this solution requires pre-planning and does not provide many of the advantages of an unplanned network mentioned above, such as scalability and efficiency.
Other known approaches to solving this problem involve modifying the routing algorithms to incorporate optical constraints and thus to ensure that the computed path is optically viable. However, as described above, photonic effects are complex and depend on both the static configuration and specifications of the optical transmission media and the dynamic state of connections provisioned across the network. As a result, such approaches tend to produce over-complex routing algorithms which are computationally intensive, if not intractable.
The ultimate objective for optimal routing is a system which addresses a set of demands at the least financial cost. However, given the above-mentioned complexity, previous work has often ignored this and instead optimised for least optical impairment as this guarantees a viable path if there is one to be found, but also optical impairment generally has some correlation with actual equipment cost. This approximation is usually valid because most equipment within a particular line system is likely to be of roughly similar performance therefore impairment is proportional to equipment used. However, where line systems are mixed (more likely in a larger photonic network), fibre types are mixed, financial arrangements intervene (e.g. rent-on-demand fibres that are much more expensive than operator owned fibres) or for many other reasons, this approximation may break down. Prior art mechanisms have been produced to perform a dual optimisation between the two factors of cost and optical viability, but these usually have fixed models of both—and often don't deliver a guaranteed optimal cost.